The internal metallic surfaces of cooling water systems, particularly ferrous surfaces, tend to pick up iron fouling, which may be compounded with scaling from hard water (calcium and magnesium scale). This fouling forms an insulating layer which not only reduces heat flow from the system (e.g., tubes) outward into the cooling medium, but also reduces volume capacity of the heat-exchanger and can promote corrosion. Hence the fouling must be removed periodically if the original design capacity of the system is to be maintained. Various cleaning procedures are known, e.g., use of hydrochloric acid (which removes Fe as soluble FeCl.sub.3), or citric acid or ammonium citrate (which removes Fe as a water-soluble complex). For either of these prior systems to be effective, the system must be shut down during cleaning, followed by water flush. The HCl-cleaning is particularly disadvantageous in this respect, since the very low pH requires vigilance to avoid damage to ferrous internals. In contrast, the instant invention functions very effectively in a near-neutral pH range (about 5-9), thereby permitting on-stream cleaning.
Certain alkylene amine carboxylic acids are known for use in removal of iron fouling in boilers. (Cf. U.S. Pat. No. Re. 30,796.) However, the problems encountered in cleaning boilers and in cleaning cooling water systems are generally markedly different. Boiler water systems operate at high water temperatures (above 220.degree. F.) and generally at high alkalinity (pH about 10 to 12). Hardness control is generally practiced. Nevertheless, the high pH and temperatures of boilers drive the residual calcium ions from the boiler water. While the calcium hardness of circulating boiler water itself is thus expected to be low, the prevalent operating conditions make boiler water systems subject to scaling with calcium-rich deposits. Moreover, oxygen levels are generally kept very low in boiler water systems to minimize oxygen induced corrosion. In contrast, many cooling water systems operate at lower water temperatures (i.e. about 70.degree. F. to about 150.degree. F, typically fluctuating between inlet water temperatures of about 90.degree.-105.degree. F., and outlet water temperatures of 140.degree.-150.degree. F.) and lower alkalinity (pH about 6 to 9.5). Cooling water systems are generally subject to at least some aeration and oxygen levels are thus relatively high. Because of the relatively low alkalinity and high oxygen concentration, cooling water systems are prone to oxidation and corrosion. Iron-rich deposits are thus formed and the iron fouling associated with cooling water systems generally includes both solids picked up from the system water and surface oxidation of metal apparatus containing iron. Moreover, waters high in calcium hardness as well as waters low in calcium hardness may generally be successfully used as cooling water; and it is common to find waters relatively high in calcium hardness circulating within a cooling water system without substantial reduction in calcium levels.
Because the iron deposits generally found in cooling systems can be substantially different in character from those found in boiler systems, the cleaning methods are generally also different. As noted in U.S. Pat. No. 4,190,463, calcium hardness (with some Fe) predominates in boilers, whereas in cooling water systems Fe predominates, with some calcium scaling. The molecular composition of the fouling complexes of course varies in the two cases, and different cleaning procedures and cleaning compositions are generally therefore employed. As is well known in the art, cleaning compounds that work in boiler systems cannot necessarily be expected to work in cooling water systems, and vice versa. For example, it is reported that alkali metal salts of ethylenediamine tetraacetic acid (EDTA) are ineffective in boiler water treatment at a pH greater than 8 (U.S. Pat. No. Re. 30,796, column 1, lines 35-40; column 3, lines 64ff; and column 4, line 9). On the other hand, EDTA and its salts give excellent results in the cooling water of the instant invention, at least where low pH and low-calcium waters are used.
Other references: U.S. Pat. No. 4,454,046 teaches treatment of boiler water with hydroxyethylethylene diaminetriacetic acid. U.S. Pat. No. 4,190,463 teaches removal of iron deposits on cooling water surfaces with hydrolyzable tanning extracts, followed by citric acid treatment. U.S. Pat. No. 3,110,679 teaches a rust-removing composition containing N,N-di-(o-hydroxybenzyl) ethylene diaminediacetic acid. U.S. Pat. No. 3,754,990 refers to N,N-di-(beta-hydroxyethyl) glycine as a chelating agent for ferrous metals. Additionally, the following U.S. patents refer to alkylene polyamine polycarboxylic acids as metal sequestrants: U.S. Pat. Nos. 3,308,065; 3,929,874; 3,965,027; 4,011,171; and 4,020,016.